A hot-working device for a steel tube that includes a steel tube feed-out device that feeds out a steel tube in its longitudinal direction, a bending support point member that guidingly supports the fed out steel tube, a high-frequency heating coil that continuously heats the steel tube, a bending device that bends the steel tube by gripping it and imparting a bending moment to the heated portion, and a cooling device that carries out a hardening treatment by quenching the bent steel tube is known from Patent Document 1 below. In accordance with this invention, since bending of a steel tube and a hardening treatment can be carried out at the same time, it is possible to obtain a high strength front side frame that has been bent into a predetermined shape.    Patent Document 1: International Application Laid-open No. WO2008/123506